Pressure measuring material

ABSTRACT

Provided is a pressure measuring material having a substrate and a pressure-sensitive layer, wherein the pressure-sensitive layer contains a polymer matrix containing a polymer compound having a molecular weight of 1,000 or more, and microcapsules encapsulating an electron-donating dye precursor and a solvent, and an electron-accepting compound. The pressure-sensitive layer has a color development-inducing layer having the electron-accepting compound and the polymer matrix and a color-developing layer having the microcapsules. A thickness of the color-developing layer is equal to or less than ½ of a thickness of the color development-inducing layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of and claims thepriority benefit of a prior application Ser. No. 17/373,812, filed onJul. 13, 2021, the disclosure of which is incorporated herein byreference in its entirety. The prior application Ser. No. 17/373,812 isa continuation application of International Application No.PCT/JP2020/001578, filed Jan. 17, 2020, the disclosure of which isincorporated herein by reference in its entirety. Further, thisapplication claims priority from Japanese Patent Application No.2019-006244, filed Jan. 17, 2019, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to a pressure measuring material and amethod for manufacturing a pressure measuring material.

2. Description of the Related Art

The pressure measuring materials (that is, the materials used formeasuring pressure) are used for a step of bonding liquid crystal glass,printing solder on a printed circuit board, adjusting pressure betweenrollers, and the like. Examples of the materials for measuring pressureinclude a pressure measuring film represented by PRESCALE (trade name;registered trademark) offered by FUJIFILM Corporation.

Various pressure measuring materials for measuring minute pressure havebeen studied.

For example, in order to obtain a visible or readable density at aminute pressure, JP2009-019949A proposes a pressure measuring materialin which a difference ΔD of density of color developed before and afterthe application of pressure of 0.05 MPa is 0.02 or more.

SUMMARY OF THE INVENTION

As seen in JP2009-019949A described above, various pressure measuringmaterials for measuring minute pressure have been studied. Meanwhile, invarious manufacturing processes, for the purpose of controlling acompression process or the like, a pressure measuring material formeasuring a pressure in a high pressure range (preferably a range of 100MPa to 10,000 MPa) is in demand.

However, because the upper limit of the pressure range that can bemeasured with the pressure measuring films on the market, that is, theupper limit of the pressure range in which color development can beobtained by the application of pressure is substantially about 300 MPa,the pressure measuring materials of the related art cannot be used formeasuring pressure, particularly, a pressure higher than 300 MPa.

As described above, the measurement of pressure in a high pressure rangeis in demand, and such a demand is met to some extend with the pressuremeasuring materials of the related art. However, a further improvementof the materials is desired.

An object of an embodiment of the present disclosure is to provide apressure measuring material from which color development with excellentgradation can be obtained in a high pressure range (preferably in arange of 100 MPa to 10,000 MPa).

The present disclosure includes the following aspects.

-   <1> A pressure measuring material having a substrate and a    pressure-sensitive layer, in which the pressure-sensitive layer    contains a polymer matrix containing a polymer compound having a    molecular weight of 1,000 or more, microcapsules encapsulating an    electron-donating dye precursor and a solvent, and an    electron-accepting compound.-   <2> The pressure measuring material described in <1>, which is in    the form of a sheet.-   <3> The pressure measuring material described in <1> or <2>, in    which an arithmetic mean roughness Ra of an outermost surface    opposite to the substrate is 10.0 µm or less.-   <4> The pressure measuring 014201material described in any one of    <1> to <3>, in which the microcapsules and the electron-accepting    compound are contained in the polymer matrix.-   <5> The pressure measuring material described in <4>, in which an    arithmetic mean roughness Ra of an outermost surface opposite to the    substrate is less than 2.0 µm.-   <6> The pressure measuring material described in <4> or <5>, in    which the electron-accepting compound includes a metal salt of    salicylic acid.-   <7> The pressure measuring material described in any one of <4> to    <6>, in which a void volume is 5 mL/m² or less.-   <8> The pressure measuring material described in any one of <4> to    <7>, in which a content of the microcapsules is 10% by volume to 80%    by volume with respect to the pressure-sensitive layer.-   <9> The pressure measuring material described in <3>, in which the    pressure-sensitive layer has a color development-inducing layer    having the electron-accepting compound and the polymer matrix and a    color-developing layer having the microcapsules, the substrate, the    color development-inducing layer, and the color-developing layer are    provided in this order, and a thickness of the color-developing    layer is equal to or less than ½ of a thickness of the color    development-inducing layer.-   <10> The pressure measuring material described in <9>, in which the    arithmetic mean roughness Ra of the outermost surface opposite to    the substrate is 2.0 to 10.0 µm.-   <11> The pressure measuring material described in <9> or <10>, in    which the electron-accepting compound includes acidic clay or    activated clay.-   <12> The pressure measuring material described in <11>, in which the    pressure-sensitive layer has inorganic particles other than the    electron-accepting compound.-   <13> The pressure measuring material described in any one of <9> to    <12>, in which a void volume is 5 mL/m² to 20 mL/m².-   <14> The pressure measuring material described in any one of <1> to    <13>, in which in a case where T represents a thickness of a layer    calculated by subtracting a thickness of the substrate from a    thickness of the pressure measuring material, and p represents an    inner diameter of the microcapsules, a ratio T/p is 1.2 or more.-   <15> The pressure measuring material described in <14>, in which in    a case where T represents the thickness of a layer calculated by    subtracting a thickness of the substrate from a thickness of the    pressure measuring material, and p represents the inner diameter of    the microcapsules, the ratio T/p is 1.2 to 5.0.-   <16> The pressure measuring material described in any one of <1> to    <15>, in which a content of the polymer compound having a molecular    weight of 1,000 or more is 10% by mass or more with respect to a    total mass of the pressure-sensitive layer.-   <17> The pressure measuring material described in any one of <1> to    <16>, in which the substrate is a polyethylene terephthalate    substrate or a polyethylene naphthalate substrate.-   <18> The pressure measuring material described in any one of <1> to    <17>, further having an easy adhesion layer between the substrate    and the pressure-sensitive layer.-   <19> The pressure measuring material described in any one of <1> to    <18>, in which a wall material of the microcapsules includes at    least one kind of material selected from polyurethane urea or    polyurethane.-   <20> A method for manufacturing the pressure measuring material    described in any one of <4> to <8> and <14> to <19>, having a step    of disposing a pressure-sensitive layer-forming composition on the    substrate, in which the pressure-sensitive layer-forming composition    contains a polymer matrix containing a polymer compound having a    molecular weight of 1,000 or more, microcapsules encapsulating an    electron-donating dye precursor and a solvent, and an    electron-accepting compound.-   <21> A method for manufacturing the pressure measuring material    described in any one of <9> to <19>, having a step of obtaining a    color-developing layer-forming composition containing a solvent and    microcapsules encapsulating an electron-donating dye precursor and a    solvent, a step of obtaining a color development-inducing    layer-forming composition containing an electron-accepting compound    and a polymer compound having a molecular weight of 1,000 or more, a    step of disposing the color development-inducing layer-forming    composition on the substrate so as to form a color    development-inducing layer, and a step of disposing the    color-developing layer-forming composition on the color    development-inducing layer so as to form a color-developing layer.

According to an embodiment of the present disclosure, it is possible toprovide a pressure measuring material from which color development withexcellent gradation can be obtained in a high pressure range (preferablyin a range of 100 MPa to 10,000 MPa).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between pressure and densityof color developed in the evaluation of color developmentcharacteristics in Examples.

FIG. 2 is a schematic cross-sectional view showing an example of apressure measuring material of the present disclosure.

FIG. 3 is a schematic cross-sectional view showing an example of apressure measuring material of the present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the pressure measuring material of the present disclosureand a manufacturing method thereof will be specifically described. Thepressure measuring material of the present disclosure and themanufacturing method thereof are not limited to the followingembodiments, and can be embodied by being appropriated modified withinthe scope of the object of the present disclosure.

In the present disclosure, a range of numerical values described using“to” means a range including the numerical values listed before andafter “to” as the lower limit and the upper limit.

Regarding the ranges of numerical values described in stages in thepresent disclosure, the upper or lower limit of a range of numericalvalues may be replaced with the upper or lower limit of another range ofnumerical values described in stages. Furthermore, regarding the rangesof numerical values described in the present disclosure, the upper orlower limit of a range of numerical values may be replaced with valuesdescribed in examples.

In the present disclosure, in a case where there is a plurality ofsubstances in a composition that corresponds to each component of thecomposition, unless otherwise specified, the amount of each component ofthe composition means the total amount of the plurality of substancespresent in the composition.

In the present disclosure, the combination of two or more preferredaspects is a more preferred aspect.

In the present disclosure, an electron-donating dye precursor is alsocalled “color-developing agent”, and an electron-accepting compound thatcauses the electron-donating dye precursor to develop color is alsocalled “developer”.

Pressure Measuring Material and Manufacturing Method Thereof

The pressure measuring material of the present disclosure has asubstrate and a pressure-sensitive layer which contains a polymer matrixcontaining a polymer compound having a molecular weight of 1,000 or more(hereinafter, also simply called “polymer matrix”), microcapsulesencapsulating an electron-donating dye precursor and a solvent, and anelectron-accepting compound. If necessary, the pressure measuringmaterial of the present disclosure may have other layers (for example, awhite layer, a protective layer, an easy adhesion layer, and the like)in addition to the substrate and the pressure-sensitive layer.

In the related art, pressure measuring materials have been proposed andwidely used. However, the pressure measuring materials of the relatedart have focused on being able to produce visible or readable densityeven by the application of minute pressure. For example, the pressuremeasuring material described in JP2009-019949A is for measuring minutepressure of less than 0.1 MPa.

In a case where the pressure measuring material is to be used formeasuring minute pressure, a broad density gradation can be obtainedfrom the material by certain effective methods (for example, the controlof particle diameter, wall thickness, and constituent materials ofmicrocapsules, and the like). However, in a case where the material isto be used for measuring pressure in a high pressure range, simply withsuch methods, sometimes it is difficult to design the material so as toobtain a broad density gradation.

As a solution to the above problem, the present disclosure provides apressure measuring material having a substrate and a pressure-sensitivelayer which contains a polymer matrix, microcapsules encapsulating anelectron-donating dye precursor and a solvent, and an electron-acceptingcompound.

From this pressure measuring material of the present disclosure, colordevelopment with excellent gradation is obtained in a high pressurerange (preferably a range of 100 MPa to 10,000 MPa, and more preferablyin a range of 300 MPa to 3,000 MPa).

The reason why the pressure measuring material of the present disclosurebrings about the above effect is unclear, but is assumed to be as belowaccording to the inventors of the present invention. Because thepressure-sensitive layer contains a polymer matrix, microcapsulesencapsulating an electron-donating dye precursor and a solvent, and anelectron-accepting compound, even though a high pressure is applied, thepressure applied to the microcapsules may be relaxed, which may make itpossible to measure pressure with excellent gradation of colordevelopment. However, the effects of the pressure measuring material ofthe present disclosure are not limited to this assumption.

In the present disclosure, “gradation of color development” means theproperties in which the density of color developed increases as thepressure applied to the pressure measuring material increases.

The pressure measuring material of the present disclosure performs colordevelopment by the contact between the electron-donating dye precursorencapsulated in the microcapsules and the electron-accepting compound asa developer in the pressure-sensitive layer. This color developmentexhibits the density corresponding to the magnitude of the externalforce applied to the pressure measuring material (pressure applied fromthe outside, the same applies hereinafter), that is, the gradation ofcolor development. For example, in a case where surface pressure isapplied to the pressure measuring material, if the applied surfacepressure is unevenly distributed throughout the surface, the materialdevelops color at a density corresponding to the pressure, and an imagehaving density gradation is obtained.

From the pressure measuring material of the present disclosure,gradation of color development can be obtained in a range of 100 MPa to10,000 MPa. In a case where a pressure less than 100 MPa and/or apressure higher than 10,000 MPa is applied to this material, gradationof color development may also be obtained.

[Substrate]

The pressure measuring material of the present disclosure has asubstrate.

The substrate may be in any shape such as a sheet shape and a plateshape. The substrate is preferably in the shape of sheet. That is, thepressure measuring material of the present disclosure is preferably asheet-shaped pressure measuring material having a sheet-shaped substrateand a pressure-sensitive layer. In the present disclosure,“sheet-shaped” means having two main surfaces, having a thickness of 1mm or less (preferably 1 µm to 1 mm), and having flexibility. In thepresent disclosure, “sheet-shaped” includes “film-shaped”, and these areused as terms having the same definition. “Plate-shaped” means havingtwo main surfaces and having a thickness more than 1 mm (preferably morethan 1 mm and 10 mm or less).

The substrate is not particularly limited. Specific examples thereofinclude paper, synthetic paper, a plastic substrate, a metal substrate,and the like. The substrate may be a composite substrate consisting ofthese. From the viewpoint of ease of handling, the substrate ispreferably a plastic substrate.

Specific examples of paper include high-quality paper, medium-qualitypaper, woody paper, alkaline paper, acidic paper, recycled paper, coatedpaper, machine-coated paper, art paper, cast-coated paper, lightlycoated paper, tracing paper, and the like.

Specific examples of plastic forming the plastic substrate includepolyesters such as polyethylene terephthalate (PET) and polyethylenenaphthalate (PEN), cellulose derivatives such as cellulose triacetate,polyolefins such as polypropylene and polyethylene, polystyrene, and thelike.

Specific examples of synthetic paper include synthetic paper (such asYUPO) obtained by biaxially stretching polypropylene or polyethyleneterephthalate so as to form many microvoids, synthetic paper preparedusing synthetic fiber such as polyethylene fiber, polypropylene fiber,polyethylene terephthalate fiber, and polyamide fiber, a laminateobtained by laminating these on a part, one surface, or both surfaces ofanother synthetic paper, and the like.

The substrate is also preferably a substrate containing a metal.Examples of the substrate of this aspect include a metal substrate, acomposite substrate consisting of a metal and plastic, and the like.

The metal is not particularly limited, but is preferably stainless steel(SUS) or the like, because such a metal is not easily deformed by themeasurement pressure.

As the plastic substrate, a polyethylene terephthalate substrate or apolyethylene naphthalate substrate is preferable, because such asubstrate can have a high hardness and a high planarity and makes itpossible to measure better pressure in a high pressure range and toobtain better density of color developed.

From the viewpoint of reproducibility of density of color developedcorresponding to pressure, a substrate is preferable which is lessdeformed by the application of pressure, does not affect a measurementobject, and can suppress pressure dispersion that causes deteriorationof measurement accuracy. As such a substrate, for example, apolyethylene naphthalate substrate or a substrate containing a metal issuitable.

Furthermore, it is also preferable to use a white substrate because thissubstrate can further improve visibility by increasing contrast betweena color-developing portion and a non-color-developing portion. As thewhite substrate, a plastic substrate is preferable, and a whitepolyethylene terephthalate substrate is more preferable. As the whitepolyethylene terephthalate substrate, a substrate obtained byincorporating a known white coloring material (for example, a whitepigment or the like) into a polyethylene terephthalate substrate may beused.

The thickness of the substrate is not particularly limited, but ispreferably 10 µm to 500 µm and more preferably 10 µm to 200 µm, becausea substrate having this thickness is easy to handle and can be suppliedin the form of a roll.

[Pressure-Sensitive Layer]

The pressure measuring material of the present disclosure has apressure-sensitive layer on the substrate.

The pressure-sensitive layer is a layer containing a polymer matrix,microcapsules encapsulating an electron-donating dye precursor and asolvent, and an electron-accepting compound.

(Polymer Matrix)

In the present disclosure, the term “polymer matrix” is used as a termfor a matrix which is a constituent of the pressure-sensitive layer andis formed of a polymer compound having a molecular weight of 1,000 ormore (hereinafter, also called a specific polymer compound).

The specific polymer compound preferably functions as a binder in thepressure-sensitive layer. The compounds constituting the microcapsules(including the inclusion of the microcapsules, the wall material, andthe dispersant used for forming the microcapsules) and theelectron-accepting compound are not included in the specific polymercompound.

In a case where the pressure-sensitive layer contains the specificpolymer compound, the polymer matrix is confirmed to be a constituent ofthe pressure-sensitive layer according to the pressure measuringmaterial of the present disclosure.

In view of further improving the gradation in a high pressure range, thecontent of a specific polymer compound with respect to the total mass ofthe pressure-sensitive layer is preferably 10% by mass or more, and morepreferably 20% by mass or more. In a case where the amount of thespecific polymer compound is 10% by mass or more, the external force(pressure) applied to the pressure measuring material is relaxed. As aresult, the pressure-sensitive layer can easily retain components suchas the microcapsules and the electron-accepting compound.

From the viewpoint of density of color developed, the content of thespecific polymer compound forming the polymer matrix with respect to thetotal mass of the pressure-sensitive layer is preferably 10% by mass to70% by mass, and more preferably 20% by mass to 50% by mass.

The specific polymer compound is not particularly limited, and can beappropriately selected depending on the properties required for thepressure-sensitive layer that is not easily deformed by the measurementpressure.

One kind of specific polymer compound may be used alone, or two or morekinds of specific polymer compounds may be used in combination.

Examples of the specific polymer compound include polyvinyl alcohol, aurethane-based polymer including polyurethane, a vinyl chloride-basedpolymer, a vinyl acetate-based polymer, an acrylic polymer,styrene-butadiene rubber (SBR), and a copolymer of these.

The urethane-based polymer means a polymer containing a constitutionalunit having a urethane bond. The vinyl chloride-based polymer means apolymer containing a constitutional unit derived from vinyl chloride.The vinyl acetate-based polymer means a polymer containing aconstitutional unit derived from vinyl acetate. The acrylic polymermeans a polymer containing a constitutional unit derived from(meth)acrylic acid.

The specific polymer compound may be incorporated into thepressure-sensitive layer in the form of a dispersion.

From the viewpoint of applicability in preparing microcapsules andproductivity in aqueous coating, polyvinyl alcohol is one of thesuitable aspects of the specific polymer compound.

The polyvinyl alcohol is not particularly limited and may beappropriately selected depending on the properties required for thepressure-sensitive layer.

In view of further improving the gradation in a high pressure range byretaining the microcapsules and the like, the degree of polymerizationof the polyvinyl alcohol is preferably 100 to 10,000, and morepreferably 100 to 3,000.

In view of further improving the gradation in a high pressure range byretaining the microcapsules and the like, the molecular weight of thespecific polymer compound is 1,000 or more, preferably 2,000 or more,more preferably 5,000 or more, and even more preferably 10,000 or more.The upper limit of the molecular weight is not particularly limited, andmay be 1,000,000 for example. From the viewpoint of ease ofmanufacturing, the molecular weight is preferably 2,000 to 100,000, morepreferably 5,000 to 100,000, and even more preferably 10,000 to 100,000.The molecular weight of the specific polymer compound represents anumber-average molecular weight measured by gel permeationchromatography (GPC).

Specifically, the aforementioned molecular weight is a molecular weightdetected using a gel permeation chromatography (GPC) analyzer usingTSKgel GMHxL, TSKgel G4000HxL, and TSKgel G2000HxL columns (trade names,manufactured by TOSOH CORPORATION), tetrahydrofuran (THF) as a solvent,and a differential refractometer, and expressed in terms of polystyreneas a standard substance.

As the specific polymer compound, commercially available products canalso be used. Examples of the commercially available products includePVA-105 (polyvinyl alcohol) and PVA-205 (polyvinyl alcohol) manufacturedby KURARAY CO., LTD.; SUPERFLEX 170 (urethane-based polymer), SUPERFLEX820 (urethane-based polymer), SUPERFLEX 830HS (urethane-based polymer),and SUPERFLEX 870 (urethane-based polymer) manufactured by DKS Co.,Ltd.; Vinyblan 287 (vinyl chloride·acrylic polymer), Vinyblan 900 (vinylchloride·acrylic polymer), Vinyblan 2684 (acrylic polymer), Vinyblan2685 (acrylic polymer), Vinyblan 2687 (acrylic polymer), and Vinyblan715S (vinyl chloride polymer) manufactured by Nissin Chemical Co., Ltd.;SUMIKAFLEX 752HQ (ethylene-vinyl acetate copolymer resin emulsion),SUMIKAFLEX 808HQ (ethylene-vinyl acetate-vinyl chloride copolymer resinemulsion), SUMIKAFLEX 850HQ (ethylene-vinyl acetate-vinyl chloridecopolymer resin emulsion), and SUMIKAFLEX 830 (ethylene-vinylacetate-vinyl chloride copolymer resin emulsion) manufactured by SumikaChemtex Co., Ltd.; Nipol LX433C (styrene butadiene rubber), NipolLX2507H (styrene butadiene rubber), Nipol LX416 (styrene butadienerubber), Nipol LX814 (acrylic polymer), and Nipol LX855EX1 (acrylicpolymer) manufactured by ZEON CORPORATION; MOVINYL 742A (acrylicpolymer), MOVINYL 1711 (acrylic polymer), MOVINYL 6520 (acrylicpolymer), MOVINYL 7980 (acrylic polymer), MOVINYL 081F (vinylacetate-ethylene-based copolymer), and MOVINYL 082 (vinylacetate-ethylene-based copolymer) manufactured by The Nippon SyntheticChemical Industry Co., Ltd.; SMARTEX SN-307R (styrene butadiene latex)manufactured by NIPPON A&L INC., and the like.

Martens hardness is an example of physical properties suitable as anindex of the specific polymer compound forming the polymer matrix.

From the viewpoint of accomplishing both the pressure measurement anddensity of color developed in a high pressure range (preferably 100 MPato 10,000 MPa, and more preferably 300 MPa to 3,000 MPa), the polymermatrix according to the present disclosure preferably contains a polymercompound having a Martens hardness of 100 N/mm² or more.

It is preferable that the polymer matrix contain a polymer compoundhaving a Martens hardness of 100 N/mm² or more, because then thedeformation of the pressure-sensitive layer is suppressed even thoughthe pressure-sensitive layer is used for measuring pressure in a highpressure range, and more accurate gradation of color development isobtained.

The Martens hardness of the polymer compound is more preferably 140N/mm² or more.

The upper limit of the Martens hardness of the polymer compound is notparticularly limited, and can be 300 N/mm² or less.

The Martens hardness can be determined using the nanoindentation methodbased on ISO 14577-1 (instrumented indentation hardness), as a valueobtained by dividing the maximum test load by the surface area of anindenter at the maximum indentation depth of the indenter. The Martenshardness can be measured, for example, using a microhardness tester suchas “HM2000” manufactured by FISCHER instruments.

The specific measurement method will be shown in Examples that will bedescribed later.

(Microcapsules)

The pressure-sensitive layer contains microcapsules encapsulating anelectron-donating dye precursor and a solvent.

Usually, each of the microcapsules has a core portion and a capsule wallfor encapsulating core materials (materials to be encapsulated (alsocalled inclusion components or inclusions)) forming the core portion.

The microcapsules each encapsulate an electron-donating dye precursorand a solvent as core materials (inclusion components). Beingencapsulated in the microcapsules, the electron-donating dye precursorcan stably exist until the microcapsules are disrupted by pressure.

-Wall Material of Microcapsule-

As the wall material of the microcapsules, water-soluble or oil-solublepolymers that have been used in the related art as wall materials ofmicrocapsules containing electron-donating dye precursors ofpressure-sensitive recording materials can be used without particularlimitations. As the wall material, among these, polyurethane urea,polyurethane, polyurea, a melamine formaldehyde resin, and gelatin arepreferable, polyurethane urea, polyurethane, polyurea, and a melamineformaldehyde resin are more preferable from the viewpoint of obtainingexcellent color development, and polyurethane urea and polyurethanehaving a urethane bond are particularly preferable.

It is preferred that the capsule wall of the microcapsules issubstantially composed of a resin. “Substantially composed of a resin”means that the content of the resin is 90% by mass or more with respectto the total mass of the capsule wall. The content of the resin ispreferably 100% by mass. That is, the capsule wall of the microcapsulesis preferably composed of a resin.

Polyurethane is a polymer having a plurality of urethane bonds, and ispreferably a reaction product generated from a raw material including apolyol and a polyisocyanate.

Polyurea is a polymer having a plurality of urea bonds, and ispreferably a reaction product generated from a raw material including apolyamine and a polyisocyanate. A part of the polyisocyanate reacts withwater and turns into polyamine. By using this properties, it is alsopossible to synthesize polyurea by using polyisocyanate without using apolyamine.

Polyurethane urea is a polymer having a urethane bond and a urea bond,and is preferably a reaction product generated from a raw materialincluding a polyol, a polyamine, and a polyisocyanate. During thereaction between a polyol and a polyisocyanate, sometimes a part of thepolyisocyanate reacts with water and turns into a polyamine, andpolyurethane urea is consequently obtained.

The melamine formaldehyde resin is preferably a reaction productgenerated by the polycondensation of melamine and formaldehyde.

The aforementioned polyisocyanate is a compound having 2 or moreisocyanate groups. Examples thereof include an aromatic polyisocyanateand an aliphatic polyisocyanate. For example, the polyisocyanate may bean adduct of a polyol such as trimethylolpropane and a difunctionalpolyisocyanate.

The aforementioned polyol is a compound having 2 or more hydroxylgroups. Examples thereof include a low-molecular-weight polyol (such asan aliphatic polyol or an aromatic polyol. “low-molecular-weight polyol”refers to a polyol having a molecular weight of 400 or less), polyvinylalcohol, a polyether-based polyol, a polyester-based polyol, apolylactone-based polyol, a castor oil-based polyol, a polyolefin-basedpolyol, a hydroxyl group-containing amine-based compound (examplesthereof include an amino alcohol, examples of the amino alcohol includeN,N,N′,N′-tetrakis(2-hydroxypropyl) ethylenediamine which is a propyleneoxide or ethylene oxide adduct of an amino compound such asethylenediamine) and the like.

The aforementioned polyamine is a compound having 2 or more amino groups(primary amino groups or secondary amino groups). Examples thereofinclude an aliphatic polyvalent amine such as diethylenetriamine,triethylenetetramine, 1,3-propylenediamine, and hexamethylenediamine; anepoxy compound adduct of an aliphatic polyvalent amine; an alicyclicpolyvalent amine such as piperazine; and a heterocyclic diamine such as3,9-bis-aminopropyl-2,4,8,10-tetraoxaspiro-(5,5)undecane.

A number-average wall thickness δ of the microcapsules depends onvarious conditions such as the type of capsule wall material and thecapsule diameter. From the viewpoint color developability in a highpressure range (preferably 100 MPa to 10,000 MPa, and more preferably300 MPa to 3,000 MPa), the number-average wall thickness δ is preferably0.02 µm to 3 µm, and more preferably 0.05 µm to 2 µm.

The wall thickness of the microcapsules refers to the thickness (µm) ofa resin film (so-called capsule wall) forming the capsule particles ofthe microcapsules. The number-average wall thickness refers to a valueobtained by measuring the thickness (µm) of capsule wall of 5microcapsules by using a scanning electron microscope (SEM) andcalculating the average thereof. Specifically, first, a certain supportis coated with a microcapsule solution and dried so that a coating filmis formed. From the obtained coating film, a cross-sectional slice isprepared, the cross section is observed using SEM, and 5 microcapsulesare randomly selected. By observing the cross section of the selectedmicrocapsules, the thickness of the capsule wall is determined, and theaverage thereof is calculated. The cross-sectional slice can also beprepared from the pressure measuring material.

-Electron-Donating Dye Precursor-

The electron-donating dye precursor is not particularly limited as longas it is a substance that develops color by donating electrons oraccepting protons of acids (hydrogen ions; H⁺). The electron-donatingdye precursor is preferably colorless. The electron-donating dyeprecursor can function as a color-developing agent.

Particularly, as the electron-donating dye precursor, a colorlesscompound is preferable which has a partial skeleton such as lactone,lactam, sultone, spiropyrane, ester, or amide that undergoes ringopening or cleavage in a case where the electron-donating dye precursorcomes into contact with the electron-accepting compound which will bedescribed later.

As the electron-donating dye precursor, it is possible to use knowncompounds used for pressure-sensitive copying paper or thermal recordingpaper. Examples of the electron-donating dye precursor include variouscompounds such as a triphenylmethanephthalide-based compound, afluoran-based compound, a phenothiazine-based compound, anindolylphthalide-based compound, a leukoauramine-based compound, arhodamine lactam-based compound, a triphenylmethane-based compound, adiphenylmethane-based compound, a triazene-based compound, aspiropyran-based compound, and a fluorene-based compound.

For details of the above compounds, JP1993-257272A (JP-H05-257272A) andparagraphs “0029” to “0034” in WO2009/008248A can be referred to.

One kind of electron-donating dye precursor may be used alone, or two ormore kinds of electron-donating dye precursors may be used incombination.

In an aspect of the present disclosure, from the viewpoint ofvisibility, the electron-donating dye precursor preferably has a highmolar light absorption coefficient (ε). The molar light absorptioncoefficient (ε) of the electron-donating dye precursor is preferably10,000 mol⁻¹·cm⁻¹·L or more, more preferably 15,000 mol⁻¹·cm⁻¹·L ormore, and even more preferably 25,000 mol⁻¹·cm⁻¹·L or more.

The molar light absorption coefficient (ε) can be calculated from theabsorbance obtained in a case where an electron-donating colorless dyeis dissolved in a 95% by mass aqueous acetic acid solution.Specifically, by using a 95% by mass aqueous acetic acid solution of anelectron-donating colorless dye having a concentration adjusted toobtain an absorbance of 1.0 or less, the molar light absorptioncoefficient (ε) can be calculated by the following equation in which A(cm) represents the length of a measurement cell, B (mol/L) representsthe concentration of the electron-donating colorless dye, and Crepresents the absorbance.

Molar light absorption coefficient(ε) = C/(A x B)

From the viewpoint of improving color developability in a pressure rangewhich is preferably 100 MPa to 10,000 MPa (more preferably 300 MPa to3,000 MPa), the content (for example, coating amount) of theelectron-donating dye precursor in the pressure-sensitive layer that isexpressed as mass after drying is preferably 0.1 g/m² to 5 g/m², morepreferably 0.1 g/m² to 4 g/m², and even more preferably 0.2 g/m² to 3g/m².

-Solvent-

The microcapsules encapsulate at least one kind of solvent. The solventcan function as an oil component that dissolves the electron-donatingdye precursor.

As the solvent, it is possible to use known solvents used forpressure-sensitive copying paper.

In the solvent, in view of stably dissolving the electron-donating dyeprecursor without forming precipitates, the content of a solvent havinga boiling point higher than 130° C. is preferably 50% by mass to 100% bymass, more preferably 70% by mass to 100% by mass, and even morepreferably 90% by mass to 100% by mass. The upper limit of the boilingpoint is not particularly limited, and may be 500° C. for example. Theupper limit of the boiling point is preferably higher than 130° C. and500° C. or lower.

Examples of the solvent include an alkylnaphthalene compound such asdiisopropyl naphthalene; a diarylalkane-based compound such as1-phenyl-1-xylyl ethane; an alkylbiphenyl-based compound such asisopropyl biphenyl; a triaryl methane-based compound; analkylbenzene-based compound; a benzyl naphthalene-based compound; adiaryl alkylene-based compound; an aromatic hydrocarbon such as arylindane-based compound; an ester-based compound such as dibutylphthalate; an aliphatic hydrocarbon such as isoparaffin; natural animaland vegetable oils such as soybean oil, corn oil, cottonseed oil,rapeseed oil, olive oil, palm oil, castor oil, and fish oil; highboiling point fractions of natural substances such as mineral oil; andthe like.

One kind of solvent may be used alone, or two or more kinds of solventsmay be used by being mixed together.

In view of color developability, the mass ratio of the solvent to theelectron-donating dye precursor encapsulated in the microcapsules(solvent:precursor) is preferably in a range of 98: 2 to 30:70, morepreferably in a range of 97: 3 to 40:60, and even more preferably in arange of 95: 5 to 50:50.

-Other Components-

If necessary, the microcapsules may encapsulate additives, in additionto the electron-donating dye precursor, solvent, and auxiliary solventdescried above. Examples of the additives include an ultravioletabsorber, a light stabilizer, an antioxidant, wax, an odor suppressant,and the like. Furthermore, the microcapsules may encapsulate a solventhaving a boiling point of 130° C. or lower (for example, a ketone-basedcompound such as methyl ethyl ketone, an ester-based compound such asethyl acetate, or an alcohol-based compound such as isopropyl alcohol)that is used for manufacturing the microcapsules.

In the pressure-sensitive layer, the content of the microcapsules(coating amount in a case where the microcapsules are provided bycoating) with respect to the total mass of solid contents of thepressure-sensitive layer is preferably 10% by mass to 80% by mass, morepreferably 10% by mass to 60% by mass, and even more preferably 10% bymass to 50% by mass.

-Method for Preparing Microcapsules-

The microcapsules can be manufactured by any of known methods such asinterfacial polymerization, internal polymerization, phase separation,external polymerization, and coacervation.

For preparing microcapsules by using polyurethane urea, polyurethane,and polyurea as a capsule wall material, for example, paragraphs “0040”to “0044” of JP2009-019949A can be referred to. Specifically, themicrocapsules can be formed, for example, by a method of mixing acompound for forming a wall material of the microcapsules with a corematerial of the microcapsules, and reacting the compound for forming awall material of the microcapsules. In forming the microcapsules, it ispreferable to use a dispersant such as polyvinyl alcohol.

(Electron-Accepting Compound)

The pressure-sensitive layer contains at least one kind ofelectron-accepting compound. The electron-accepting compound canfunction as a developer.

Examples of the electron-accepting compound include an inorganiccompound and an organic compound.

Specific examples of the inorganic compound include acidic clay,activated clay, attapulgite, zeolite, bentonite, a clay substance suchas kaolin, and the like.

Specific examples of the organic compound include a metal salt of anaromatic carboxylic acid (preferably a metal salt of salicylic acid), aphenol formaldehyde resin, a metal salt of a carboxylated terpene phenolresin, and the like.

Among these, acidic clay, activated clay, zeolite, kaolin, a metal saltof aromatic carboxylic acid, or a metal salt of a carboxylated terpenephenol resin is preferable as the electron-accepting compound, andacidic clay, activated clay, kaolin, or a metal salt of an aromaticcarboxylic acid is more preferable.

Specifically, as the aromatic carboxylic acid in the metal salt ofaromatic carboxylic acid, for example, 3,5-di-t-butylsalicylic acid,3,5-di-t-octylsalicylic acid, 3,5-di-t-nonylsalicylic acid,3,5-di-t-dodecylsalicylic acid, 3-methyl-5-t-dodecylsalicylic acid,3-t-dodecylsalicylic acid, 5-t-dodecylsalicylic acid,5-cyclohexylsalicylic acid, 3,5-bis (α,α-dimethylbenzyl)salicylic acid,3-methyl-5-(α-methylbenzyl)salicylic acid,3-(α,α-dimethylbenzyl)-5-methylsalicylic acid,3-(α,α-dimethylbenzyl)-6-methylsalicylic acid,3-(α-methylbenzyl)-5-(α,α-dimethylbenzyl)salicylic acid,3-(α,α-dimethylbenzyl)-6-ethylsalicylic acid, and3-phenyl-5-(α,α-dimethylbenzyl)salicylic acid are preferable.Furthermore, a carboxy-modified terpene phenol resin, a salicylic acidresin which is a reaction product of 3,5-bis(α-methylbenzyl)salicylicacid and benzyl chloride, and the like can also be used as the aromaticcarboxylic acid. Specific examples of the metal salt in the metal saltof an aromatic carboxylic acid include a zinc salt, a nickel salt, analuminum salt, and a calcium salt.

The content of the electron-accepting compound in the pressure-sensitivelayer (coating amount in a where the electron-accepting compound isprovided by coating) that is expressed as dry mass is preferably 0.1g/m² to 30 g/m². In a case where the electron-accepting compound is aninorganic compound, the content thereof expressed as dry mass is morepreferably 3 g/m² to 20 g/m², and more preferably 5 g/m² to 15 g/m². Ina case where the electron-accepting compound is an organic compound, thecontent thereof expressed as dry mass is more preferably 0.1 g/m² to 15g/m², and even more preferably 0.2 g/m² to 10 g/m².

(Oil-Absorbing Particles)

The pressure-sensitive layer preferably contains at least one kind ofoil-absorbing particles outside the microcapsules.

Because high pressure is applied to the pressure measuring material ofthe present disclosure, the solvent (oil component) encapsulated in themicrocapsules tends to easily leach out of the pressure-sensitive layer.Leaking of the oil component is not desirable because it can cause oilstains. However, in a case where the pressure-sensitive layer containsoil-absorbing particles on the outside of the microcapsules, the leakageof the oil component out of the pressure-sensitive layer can beeffectively suppressed.

In the present disclosure, “oil-absorbing particles” mean particles thatabsorb linseed oil at 25° C. until an oil absorption amount reaches 50%by mass or more of the weight of the particles.

The oil absorption amount is measured according to JIS-K5101-13-1: 2004.

Examples of the shape of the particles include a spherical shape, anelliptical shape, a rod shape, and the like. The particles may haveother shapes.

The particle diameter of the oil-absorbing particles is preferably 0.5µm to 20 µm, more preferably 1 µm to 10 µm, and even more preferably 2µm to 8 µm.

The particle diameter of the oil-absorbing particles can be measuredusing Microtrac MT3300EXII (manufactured by NIKKISO CO., LTD.).

Examples of the oil-absorbing particles include inorganic particles suchas porous silica particles, calcium carbonate, kaolin, aluminumsilicate, calcium silicate, colloidal silica, alumina, and aluminumhydroxide, and polymer particles such as polyolefin, acryl, polystyrene,and polyester. Among these, at least one kind of inorganic particlesselected from porous silica particles, calcium carbonate, or kaolin arepreferable.

Among the electron-accepting compounds as developers, an oil-absorbingcompound may be used as the oil-absorbing particles.

As the oil-absorbing particles, commercially available products may beused. Examples thereof include “BRILLIANT Series” manufactured bySHIRAISHI KOGYO KAISHA, LTD., and the like.

The content of the oil-absorbing particles in the pressure-sensitivelayer can be appropriately set depending on the desired oil-absorbingproperties.

(Inorganic Particles)

The pressure-sensitive layer preferably contains at least one kind ofinorganic particles not being an electron-accepting compound outside themicrocapsules.

Examples of the inorganic particles include porous silica particles,calcium carbonate, kaolin, aluminum silicate, calcium silicate,colloidal silica, alumina, aluminum hydroxide, and the like. Amongthese, silica is preferable. The inorganic particles may be inorganicparticles as the oil-absorbing particles described above.

As the inorganic particles, commercially available products may be used.Examples thereof include “MIZUKASIL series” manufactured by MIZUSAWAINDUSTRIAL CHEMICALS, LTD., and the like.

The particle diameter of the inorganic particles is preferably 1 µm to30 µm, and more preferably 5 µm to 20 µm.

(Other Components)

The pressure-sensitive layer may contain other components. Examplesthereof include a surfactant, a fluorescent brightener, an antifoamingagent, a penetrant, an ultraviolet absorber, a preservative, and thelike.

(Thickness T of Pressure-Sensitive Layer)

A thickness t of the pressure-sensitive layer is not particularlylimited and can be selected depending on the purpose and the like.

The thickness t of the pressure-sensitive layer is preferably 1 µm to250 µm, more preferably 3 µm to 200 µm, even more preferably 5 µm to 150µm, and particularly preferably 5 µm to 50 µm.

In a case where the pressure-sensitive layer has a color-developinglayer and a color development-inducing layer as in the second aspectwhich will be described later, the total thickness of thecolor-developing layer and the color development-inducing layer isregarded as the thickness of the pressure-sensitive layer.

The thickness t of the pressure-sensitive layer can be measured bymicroscopy.

Specifically, a pressure measuring material as a measurement object isvertically cut to prepare a cross-sectional slice, the cross-sectionalslice is observed using a scanning electron microscope (SEM), and thethickness of the pressure-sensitive layer can be obtained from theimage. Examples of the scanning electron microscope include a desktopmicroscope “Miniscope TM3030Plus” (manufactured by Hitachi High-TechCorporation.).

The thickness of the pressure-sensitive layer in the present disclosureis an arithmetic mean of thicknesses at 10 sites that are randomlyselected.

The coefficient of variation (hereinafter, also called CV value) of theparticle diameter distribution of all the particles contained in thepressure-sensitive layer is preferably 20% to 150%.

In a case where the CV value is within the above range, the particledistribution in the pressure-sensitive layer, particularly, the relativevariation of the microcapsules is small. Therefore, excellent colordevelopability is obtained.

The CV value is preferably 20% to 110%, and more preferably 25% to 80%.

The CV value represents the relative variation of the particlescontained in the pressure-sensitive layer, which is a value calculatedas below.

$\begin{array}{l}{\text{CV value}(\%) = {\text{standard deviation}/\text{arithmetic mean particle}}} \\{\text{diameter} \times \text{100}}\end{array}$

The arithmetic mean particle diameter and the standard deviation arevalues calculated by imaging the surface of the pressure-sensitive layerwith an optical microscope at a 150X magnification, and measuring thesize of all microcapsules in a 2 cm x 2 cm area that is randomly set.

(Layer Constitution of Pressure-Sensitive Layer)

The pressure-sensitive layer of the present disclosure may beconstituted with one layer or multiple layers.

In an aspect of the layer constitution of the pressure-sensitive layer,the pressure-sensitive layer can be constituted with a layer in whichmicrocapsules and an electron-accepting compound are contained in apolymer matrix. The pressure-sensitive layer adopting this aspect willbe specifically described with reference to the pressure-sensitive layerof the first aspect that will be described later as an example.

In another aspect of the pressure-sensitive layer of the presentdisclosure, the pressure-sensitive layer can have a colordevelopment-inducing layer having an electron-accepting compound and apolymer matrix and a color-developing layer having microcapsules. Thepressure-sensitive layer adopting this aspect will be specificallydescribed with reference to the pressure-sensitive layer of the secondaspect that will be described later as an example.

First Aspect

The first aspect of the pressure-sensitive layer of the presentdisclosure is a pressure-sensitive layer in which microcapsules and anelectron-accepting compound are contained in a polymer matrix.

The details of components (such as a specific polymer compound,microcapsules, and an electron-accepting compound) used in thepressure-sensitive layer according to the first aspect are the same asthose described above, and the preferred aspects of the components arealso the same as those described above.

In the present disclosure, the state where a certain component is“contained in a polymer matrix” means that at least a part of thecomponent is contained in the pressure-sensitive layer, in a state ofbeing in contact with the specific polymer compound. The componentcontained in the polymer matrix may be in a dispersed state or dissolvedstate. In a case where the component is a solid, a part thereof may beexposed on the surface of the pressure-sensitive layer.

Specifically, in a case where the component is microcapsules, a statewhere the entirety of the microcapsules is on the inside of thepressure-sensitive layer and a state where a part of the capsule wall ofthe microcapsules is exposed on the surface of the pressure-sensitivelayer are both regarded as a state where the microcapsules in thepresent disclosure are contained in the polymer matrix. A state wherethe entirety of the microcapsules is in contact with the specificpolymer compound includes both a state where the microcapsules eachconsisting of inclusions and a wall material are in direct contact withthe specific polymer compound and a state where the microcapsules are incontact with the specific polymer compound via a dispersant.

An example of the pressure measuring material having thepressure-sensitive layer according to the first aspect will be describedwith reference to FIG. 2 as appropriate.

FIG. 2 is a schematic cross-sectional view showing an example of thepressure measuring material having the pressure-sensitive layeraccording to the first aspect. Note that FIG. 2 is used to describe thefirst aspect, and the size and ratio of the constituents shown in thedrawing are not the same as the actual ones.

In a pressure measuring material 10 shown in FIG. 2 , apressure-sensitive layer 14 is provided on a substrate 12. Thepressure-sensitive layer 14 has microcapsules 18 and anelectron-accepting compound 15. 18 a represents a capsule wall of themicrocapsules and 18 b represents inclusions (that is, core materials)of the microcapsules. 16 represents a polymer matrix containing thespecific polymer compound (not shown in the drawing).

The pressure measuring material having the pressure-sensitive layeraccording to the first aspect can be particularly preferably used in apressure range of 500 MPa or more.

From the viewpoint of excellent gradation in a high pressure range of500 MPa or higher, the arithmetic mean roughness Ra of the pressuremeasuring material according to the first aspect is preferably less than2.0 µm. In other words, it is preferable that most of the microcapsulesand the electron-accepting compound be located in the polymer matrixwithout being exposed on the surface of the pressure-sensitive layer.

In the present disclosure, the arithmetic mean roughness Ra of thepressure measuring material means the arithmetic mean roughness Raspecified in JIS B 0681-6: 2014. As a measurement device, a white lightscanning interferometer using optical interferometry (specifically,NewView5020 manufactured by Zygo: Stich mode; objective lens with a 50Xmagnification; intermediate lens with a 0.5X magnification) is used.

The arithmetic mean roughness Ra corresponds to the arithmetic meanroughness Ra of the surface opposite to the substrate side. Thearithmetic mean roughness Ra of the pressure measuring materialaccording to the first aspect is preferably 0 µm or more and less than2.0 µm, more preferably 0 µm to 1.0 µm, and even more preferably 0 µm to0.5 µm.

Examples of the method for obtaining the arithmetic mean roughness Ra ofless than 2.0 µm include increasing the amount of the specific polymercompound in the pressure-sensitive layer, and the like. The amount ofthe specific polymer compound is preferably 20% by mass or more withrespect to the total solid content constituting the pressure-sensitivelayer.

From the viewpoint of excellent gradation in a high pressure range of500 MPa or higher, the pressure measuring material having thepressure-sensitive layer according to the first aspect preferably has avoid volume of 5 mL/m² or less. In a case where the void volume is low,the microcapsules easily endure pressure and are hardly disrupted untilthe applied pressure reaches 500 MPa or higher. The void volume ispreferably 0 mL/m² to 5 mL/m², more preferably 0 mL/m² to 3 mL/m², andeven more preferably 0 mL/m² to 1 mL/m².

The void volume is a value calculated by the following equation.

A mass (m₁) of the pressure measuring material cut in 10 cm × 10 cm ismeasured. Then, the pressure measuring material is permeated withdiethylene glycol from the surface provided with the pressure-sensitivelayer, the diethylene glycol remaining on the surface is wiped off, andthen a mass (m₂) is measured. Thereafter, m₂ - m₁.is calculated to findX, and a void volume is calculated from the following equation. Thedensity of the diethylene glycol is 1.118.

Void volume(m₁/m₂) = 100 × X ÷ 1.118

In view of reducing the void volume, the electron-accepting compoundcontained in the pressure-sensitive layer of the first aspect preferablycontains an organic compound, more preferably contains an organiccompound as a main component, and even more preferably has an organiccompound content of 50% by mass to 100% by mass. As theelectron-accepting compound, for example, the compounds described aboveare preferable. The electron-accepting compound preferably includes ametal salt of an aromatic carboxylic acid, and particularly preferablyincludes a metal salt of salicylic acid. In the present specification,“containing a compound as a main component” means that the compound is acomponent of the highest content among electron-accepting compounds.

In view of further improving gradation in a high pressure range, thecontent (volume fraction) of the microcapsules with respect to thepressure-sensitive layer is preferably 10% by volume to 80% by volume,more preferably 20% by volume to 60% by volume, and even more preferably30% by volume to 60% by volume.

The content (volume fraction) of the microcapsules with respect to thepressure-sensitive layer can be measured by the following method. Themethod will be described with reference to FIG. 2 .

A cross-sectional slice of the pressure measuring material 10 isprepared, and the cross-section is observed with a scanning electronmicroscope (SEM) at a 1,000X magnification. In the SEM image of thecross section, the inside of microcapsules 18 (inclusion 18b) and amatrix portion including a capsule wall 18 a, a polymer matrix 16containing the specific polymer compound (not shown in the drawing), andthe electron-accepting compound 15 are observed as parts distinguishedfrom each other. For all the microcapsules 18 existing in the observedfield of view, the inside of the microcapsules 18 (inclusion 18b) andthe matrix portion including the capsule wall 18 a, the polymer matrix16 containing the specific polymer compound, and the electron-acceptingcompound 15 are separated by image analysis. The internal area of themicrocapsules and the area of the matrix portion are calculated, andfrom the ratio thereof, a content A (% by area) of the inside of themicrocapsules is calculated. Next, another cross-sectional slice isprepared which is cut in a direction orthogonal to the aforementionedcross-sectional slice and to the substrate, and a content B (% by area)of the microcapsules is calculated in the same manner as describedabove. The average of the content A (% by area) of the inside of themicrocapsules and the content B (% by area) of the inside of themicrocapsules is calculated. This operation is performed at two sitesthat are randomly selected, and the averages obtained at the two sitesare averaged and adopted as the content (% by volume) of themicrocapsules.

-Particle Diameter D₁ of Microcapsules-

In the first aspect, “particle diameter d₁ of microcapsules” means avolume-based median diameter.

In a case where all the microcapsules contained in thepressure-sensitive layer are divided into two groups based on athreshold value that is a particle diameter at which the cumulativevolume is 50%, a diameter (D50) at which the total volume of particlesin the large diameter group equals the total volume of particles in thesmall diameter group is the volume-based median diameter of themicrocapsules.

The volume-based median diameter of the microcapsules is obtained bycoating a support with a microcapsule solution, imaging the surface of acoating film formed after drying by using an optical microscope at a150X magnification, measuring the size of all the microcapsules in a 2cm × 2 cm area, and calculating the volume-based median diameter.

From the viewpoint of color developability in a high pressure range(preferably 100 MPa to 10,000 MPa, and more preferably 300 MPa to 3,000MPa), the particle diameter d₁ of the microcapsules is preferably 1 µmto 50 µm, and more preferably 5 µm to 30 µm.

From the viewpoint of measuring higher pressure and obtaining a higherdensity of color developed, the ratio of the thickness t of thepressure-sensitive layer to the particle diameter d₁ of themicrocapsules contained in the pressure-sensitive layer preferablysatisfies the relationship shown in Expression 1.

$\begin{matrix}{1 < {\text{t}/\text{d}_{1}} < 5} & \text{­­­Expression 1}\end{matrix}$

In a case where t/d₁ < 5 is satisfied, better color developability canbe obtained. In a case where 1 < t/d₁ is satisfied, fogging can beeasily suppressed.

-Inner Diameter P₁ of Microcapsules -

From the viewpoint of further improving gradation in a high pressurerange and from the viewpoint of color developability in a high pressurerange, an inner diameter p₁ of the microcapsule is preferably 0.5 µm to50 µm, more preferably 1 µm to 30 µm, and even more preferably 2 µm to20 µm.

In the first aspect, “inner diameter p₁ of the microcapsules” is a valueobtained by the following method. The method will be described withreference to FIG. 2 .

A cross-sectional slice of the pressure measuring material 10 isprepared, and the cross-section is observed with a scanning electronmicroscope (SEM) at a 1,000X magnification. In the SEM image of thecross section, the inside of microcapsules 18 (inclusion 18 b), acapsule wall 18 a, and a matrix portion including a polymer matrix 16containing the specific polymer compound and the electron-acceptingcompound 15 are observed as parts distinguished from each other. Amongthe microcapsules 18 existing in the observed field of view, the majoraxis (inner diameter) of 10 microcapsules is measured in order from thelargest microcapsule, and an arithmetic mean thereof is calculated toobtain an average. This operation is performed in 5 fields of view, theaverages obtained at the respective sites are further averaged, and theobtained value is adopted as the average inner diameter of themicrocapsules. The major axis means the longest inner diameter of themicrocapsules observed.

Second Aspect

The second aspect of the pressure-sensitive layer of the presentdisclosure is a pressure-sensitive layer having a colordevelopment-inducing layer containing an electron-accepting compound anda polymer matrix and a color-developing layer containing microcapsules,in which a substrate, the color development-inducing layer, and thecolor-developing layer are provided in this order, and the thickness ofthe color-developing layer is equal to or less than ½ of the thicknessof the color development-inducing layer.

The details of components (such as a specific polymer compound,microcapsules, and an electron-accepting compound) used in thepressure-sensitive layer according to the second aspect are the same asthose described above, and the preferred aspects of the components arealso the same as those described above.

An example of the pressure measuring material having thepressure-sensitive layer according to the second aspect will bedescribed with reference to a drawing.

FIG. 3 is a schematic cross-sectional view showing an example of thepressure measuring material having the pressure-sensitive layeraccording to the second aspect. Note that FIG. 3 is used to describe thesecond aspect, and the size and ratio of the constituents shown in thedrawing are not the same as the actual ones.

In a pressure measuring material 20 shown in FIG. 3 , apressure-sensitive layer 24 is provided on a substrate 22. Thepressure-sensitive layer 24 is formed of a color-developing layer 24 aand a color development-inducing layer 24 b. The color-developing layer24 a has microcapsules 28. The color development-inducing layer 24 b hasa polymer matrix 26 containing an electron-accepting compound 25 and thespecific polymer compound. 28 a represents the capsule wall of themicrocapsules, and 28 b represents inclusions (that is, core materials)of the microcapsules. The color-developing layer 24 a preferablycontains the specific polymer compound.

From the viewpoint of further improving gradation in a high pressurerange of 100 MPa or higher, the thickness of the color-developing layeris preferably equal to or less than ½ and more preferably equal to orless than ⅓ of the thickness of the color development-inducing layer.The lower limit is not particularly limited. In an aspect of the presentdisclosure, the thickness of the color-developing layer is preferably0.1% to 50%, more preferably 0.1% to 40%, and even more preferably 0.1%to 33% of the thickness of the color development-inducing layer.

The thickness of the color-developing layer and the colordevelopment-inducing layer can be measured by microscopy.

Specifically, a cross-sectional slice is prepared by vertically cuttingthe pressure measuring material as a measurement object. In thiscross-sectional slice, a field of view of 800 µm × 600 µm is observedwith a scanning electron microscope (SEM). From the image, thethicknesses of the color-developing layer and the colordevelopment-inducing layer are measured at 10 sites at intervals of 50µm, the arithmetic mean thereof is calculated, and the obtained value isadopted as the thickness of the color-developing layer and the colordevelopment-inducing layer. Examples of the scanning electron microscopeinclude a desktop microscope “Miniscope TM3030Plus” (manufactured byHitachi High-Tech Corporation.).

As shown in FIG. 3 , in a case where the color-developing layer has asite devoid of microcapsules, the thickness of the color-developinglayer is regarded as 0 µm.

The second aspect can be particularly preferably used in a pressurerange of 100 MPa to 500 MPa. From the viewpoint of excellent gradationin a high pressure range of 100 MPa to 500 MPa, the void volume of thepressure measuring material having the pressure-sensitive layeraccording to the second aspect is preferably 5 mL/m² to 20 mL/m², andmore preferably more than 8 mL/m² and 15 mL/m² or less. The void volumeis a value calculated by the equation described above.

In order that the void volume falls into a specific range, theelectron-accepting compound in the second aspect preferably containsinorganic particles, more preferably contains inorganic particles as amain component, and even more preferably contains inorganic particles ata content of 50% by mass to 100% by mass. As the electron-acceptingcompound, for example, the electron-accepting compounds described aboveare preferable. The electron-accepting compound preferably includesacidic clay or activated clay. The electron-accepting compound in thesecond aspect may mainly consists of inorganic particles or may includeother electron-accepting compounds.

From the viewpoint of excellent gradation in the high pressure range of100 MPa to 500 MPa, the pressure-sensitive layer in the second aspectpreferably contains inorganic particles other than theelectron-accepting compound. Examples of inorganic particles other thanthe electron-accepting compound include the inorganic particlesdescribed above. As the inorganic particles, silica is preferable.

From the viewpoint of further improving gradation in a high pressurerange of 100 MPa to 500 MPa, the pressure-sensitive layer (preferablythe color development-inducing layer) in the second aspect preferablycontains inorganic particles as an electron-accepting compound andinorganic particles not being an electron-accepting compound. In a casewhere the pressure-sensitive layer contains both the inorganic particlesas an electron-accepting compound and inorganic particles not being anelectron-accepting compound, the void volume can be kept in a specificrange, and the probability that the electron-donating dye precursorhaving leaked out of the microcapsules may come into contact with theelectron-accepting compound can be reduced. Accordingly, it is possibleto obtain a material having suitable gradation in a high pressure rangeof 100 MPa to 500 MPa.

From the viewpoint of excellent gradation in a high pressure range of100 MPa to 500 MPa, the arithmetic mean roughness Ra of the pressuremeasuring material according to the second aspect is preferably 2.0 µmto 10.0 µm.

The arithmetic mean roughness Ra corresponds to the arithmetic meanroughness Ra of the surface opposite to the substrate side. Thearithmetic mean roughness Ra of the pressure measuring materialaccording to the second aspect is preferably 2.0 µm to 8.0 µm, and morepreferably 2.0 µm to 5.0 µm.

The arithmetic mean roughness Ra can be adjusted to 2.0 µm to 10.0 µm,for example, by the following methods (1) and (2) and a method as acombination of these.

Method of Thinning Color-Developing Layer

Because the microcapsules are arranged not on the entire surface of thepressure-sensitive layer, the surface of the pressure-sensitive layerhas sites where the microcapsules are present and sites where themicrocapsules are absent. By using this fact, the method (1) can adjustthe surface roughness.

Method of Increasing Amount of Inorganic Particles Contained in ColorDevelopment-Inducing Layer

This is a method of adjusting the surface roughness of the colordevelopment-inducing layer by utilizing the difference between the sitewhere inorganic particles are present and the site where inorganicparticles are absent.

Particularly, in a case where the amount of inorganic particles (totalamount of inorganic particles as an electron-accepting compound andinorganic particles not being an electron-accepting compound) is largerthan the total amount of the specific polymer compound in the colordevelopment-inducing layer, the particles are exposed on the surface ofthe color development-inducing layer. Therefore, the colordevelopment-inducing layer is likely to be a rough surface. Furthermore,in a case where the microcapsules and the specific polymer compound aredisposed as a thin layer on the color development-inducing layer, themicrocapsules are stuck in the recesses of the rough colordevelopment-inducing layer. As a result, it is easy to obtain a pressuremeasuring material having the arithmetic mean roughness Ra of 2.0 µm to10.0 µm.

In such a material having an arithmetic mean roughness Ra of 2.0 µm to10.0 µm, some microcapsules are stuck in recesses of the colordevelopment-inducing layer and thus not disrupted even under highpressure conditions, or microcapsules are absent in some regions withinthe surface. It is considered that for this reason, the pressuremeasuring material of the second aspect can be suited for high pressurerange of 100 MPa to 500 MPa.

In the second aspect, it is preferable that there be no microcapsules onthe entire surface of the pressure-sensitive layer opposite to thesubstrate side. The proportion of the microcapsules is preferably 95% byarea or less, and more preferably 90% by area or less.

The proportion of the microcapsules can be measured, for example, by amethod of first observing the material from the surface at any positionof the color-developing layer by using a laser microscope (KEYENCEVK-8510, size of field of view: 100 µm × 150 µm), counting all themicrocapsules observed in the field of view, calculating the area of thecounted microcapsules observed in the field of view by image analysis,and dividing the calculated area by the area of the field of view.

From the viewpoint of facilitating the microcapsules to be stuck in therecesses of the color development-inducing layer and obtaining excellentgradation in a high pressure range of 100 MPa to 500 MPa, the amount oftotal solid content of a color-developing layer-forming composition ispreferably smaller than the amount of total solid content of a colordevelopment-inducing layer-forming composition. The amount of totalsolid content of the color-developing layer-forming composition is morepreferably 0.1% to 45% and more preferably 0.5% to 25% of the amount oftotal solid content of the color development-inducing layer-formingcomposition.

-Particle Diameter D₂ of Microcapsules-

In the second aspect, “particle diameter d₂ of microcapsules” means anaverage particle diameter.

The average particle diameter of the microcapsules is determined by amethod of imaging the surface of the color-developing layer wheremicrocapsules are present by using an optical microscope (OLYMPUS BX60,size of field of view: 320 µm × 450 µm), performing image analysis onthe obtained image, measuring the major axis (particle diameter) of 30microcapsules in order from the largest microcapsules, and calculatingthe arithmetic mean thereof to obtain an average. This operation isperformed at any 5 sites (5 fields of view) in a first layer, theaverages obtained at the respective sites are further averaged, and theobtained value is adopted as the average particle diameter of themicrocapsules. The major axis means the longest diameter of themicrocapsules observed.

From the viewpoint of color developability in a high pressure range(preferably 100 MPa to 10,000 MPa, and more preferably 300 MPa to 3,000MPa), the particle diameter d₂ of the microcapsules is preferably 1 µmto 50 µm, and more preferably 5 µm to 30 µm.

-Inner Diameter P₂ of Microcapsules-

From the viewpoint of further improving gradation in a high pressurerange of 100 MPa to 500 MPa and from the viewpoint of colordevelopability in a high pressure range, an inner diameter p₂ of themicrocapsules is preferably 1 µm to 50 µm, more preferably 2 µm to 20µm, and even more preferably 2 µm to 15 µm.

In the second aspect, “inner diameter p₂ of the microcapsules” is avalue obtained by the following method.

First, the wall thickness of the microcapsules is determined. The wallthickness of the microcapsules refers to the thickness (µm) of a capsulewall forming the capsule particles of the microcapsules. Thenumber-average wall thickness refers to a value obtained by measuringthe thickness (µm) of the capsule wall of 5 microcapsules by using ascanning electron microscope (SEM) and calculating the average thereof.More specifically, a cross-sectional slice of the microcapsules existingin the pressure measuring material is prepared, the cross section isobserved with SEM at a 15,000X magnification, and 5 microcapsules arerandomly selected which have major axis in a range of (average particlediameter of microcapsules) × 0.9 to (average particle diameter ofmicrocapsules) x 1.1. Then, the cross section of each of the selectedmicrocapsules is observed, the capsule wall thickness is determined, andthe average thereof is calculated. The major axis means the longestdiameter of the microcapsules observed. The inner diameter of themicrocapsules is calculated by average particle diameter - (wallthickness of microcapsules x 2).

Formation of Pressure-Sensitive Layer

The formation of the pressure-sensitive layer is not particularlylimited as long as the pressure-sensitive layer is formed by a step offorming a pressure-sensitive layer containing a polymer matrixcontaining a polymer compound having a molecular weight of 1,000 or more(specific polymer compound), microcapsules encapsulating anelectron-donating dye precursor and a solvent, and an electron-acceptingcompound.

In the first aspect, the pressure-sensitive layer can be formed bypreparing a pressure-sensitive layer-forming composition, applying thecomposition to a substrate (for example, by means of coating), anddrying the composition.

That is, the pressure measuring material having the pressure-sensitivelayer of the first aspect is preferably obtained by a manufacturingmethod having a step of disposing a pressure-sensitive layer-formingcomposition on a substrate, in which the composition contains a polymermatrix containing a specific polymer compound, microcapsulesencapsulating an electron-donating dye precursor and a solvent, and anelectron-accepting compound.

The pressure-sensitive layer-forming composition used for forming thepressure-sensitive layer of the first aspect can be prepared, forexample, by preparing a microcapsule dispersion, and mixing the obtaineddispersion with a solution (or emulsion) of a specific polymer compound(polymer compound forming a polymer matrix), an electron-acceptingcompound, and other optional components (for example, oil-absorbingparticles and the like).

In the second aspect, the pressure-sensitive layer can be formed, forexample, by preparing 2 kinds of compositions consisting of a colordevelopment-inducing layer-forming composition and a color-developinglayer-forming composition as a pressure-sensitive layer-formingcomposition, applying the color development-inducing layer-formingcomposition to a substrate (for example, by means of coating), applyingthe color-developing layer-forming composition thereon (for example, bymeans of coating), and drying the compositions. The color-developinglayer-forming composition can be prepared, for example, by preparing amicrocapsule dispersion, mixing the obtained dispersion with a solution(or emulsion) of a specific polymer compound and other optionalcomponents (for example, a surfactant and the like). The colordevelopment-inducing layer-forming composition can be prepared, forexample, by mixing together an electron-accepting compound, a solution(or emulsion) of a specific polymer compound, and other optionalcomponents (for example, inorganic particles and the like).

That is, the pressure measuring material having the pressure-sensitivelayer of the second aspect is preferably obtained by a manufacturingmethod including a step of obtaining a color-developing layer-formingcomposition containing a solvent (preferably a solvent having a boilingpoint of 130° C. or lower) and microcapsules encapsulating anelectron-donating dye precursor and a solvent (preferably a solventhaving a boiling point of 130° C. or higher), a step of obtaining acolor development-inducing layer-forming composition containing anelectron-accepting compound and a polymer compound (specific polymercompound) having a molecular weight of 1,000 or more, a step ofdisposing the color development-inducing layer-forming composition on asubstrate so as to form a color development-inducing layer, and a stepof disposing the color-developing layer-forming composition on the colordevelopment-inducing layer so as to form a color-developing layer.

The color-developing layer-forming composition preferably contains apolymer compound (specific polymer compound) having a molecular weightof 1,000 or more.

The color development-inducing layer-forming composition may contain onekind of specific polymer compound or two or more kinds of specificpolymer compounds in combination. The color-developing layer-formingcomposition may contain one kind of specific polymer compound or two ormore kinds of specific polymer compounds in combination. Furthermore,the specific polymer compounds contained in the colordevelopment-inducing layer-forming composition and the color-developinglayer-forming composition may be the same polymer compounds or differentpolymer compounds.

In forming the pressure-sensitive layer, the specific preparationmethod, coating amount, and drying conditions of the pressure-sensitivelayer-forming composition and the like may be appropriately determineddepending on the type of components to be incorporated into thepressure-sensitive layer-forming composition, the specific aspect of thetargeted pressure measuring material, and the like.

In a case where the pressure-sensitive layer is formed by coating asubstrate with the pressure-sensitive layer-forming composition, thecoating can be performed by known coating methods. Examples of thecoating method include coating methods using an air knife coater, a rodcoater, a bar coater, a curtain coater, a gravure coater, an extrusioncoater, a die coater, a slide bead coater, a blade coater, and the like.

[Other Layers]

The pressure measuring material of the present disclosure may have otherlayers on the substrate in addition to the pressure-sensitive layer.

Examples of those other layers include a protective layer, a whitelayer, an easy adhesion layer, and the like.

-Protective Layer-

The pressure measuring material of the present disclosure mayadditionally have a protective layer on a side of the pressure-sensitivelayer opposite to the other side provided with the substrate. Thepressure measuring material of the present disclosure can have aprotective layer as an outermost layer, but is not limited to thisaspect.

Because high pressure is applied to the pressure measuring material ofthe present disclosure, the solvent (oil component) encapsulated in themicrocapsules tends to easily leach out of the pressure-sensitive layer.Leaking of the oil component is not desirable because it can cause oilstains. In a case where the pressure measuring material has a protectivelayer, it is possible to effectively inhibit the oil component fromleaking out of the pressure-sensitive layer.

Therefore, the protective layer is preferably a layer that exhibits lowpermeability to the oil component.

The protective layer can be provided by sticking a sheet or film forforming a protective layer onto the pressure-sensitive layer.

In a case where the protective layer is to be provided by sticking thesheet or film for forming a protective layer, a desired sheet or filmfor forming a protective layer may be prepared and stuck onto thepressure-sensitive layer by known methods (for example, sticking usingan adhesive and the like).

The thickness of the protective layer is not particularly limited, andcan be selected depending on the purpose and the like.

The thickness of the protective layer is preferably 0.1 µm to 50 µm, andmore preferably 0.5 µm to 10 µm.

-White Layer-

The pressure measuring material of the present disclosure mayadditionally have a white layer between the substrate and thepressure-sensitive layer.

In a case where the material has a white layer, it is possible toincrease the contrast between a color-developing portion and anon-color-developing portion and improve visibility.

The white layer may be a coating layer formed between the substrate andthe pressure-sensitive layer by using a composition for forming a whitelayer, or a layer provided by sticking a sheet or film for forming awhite layer onto the substrate before the pressure-sensitive layer isprovided.

The white layer can be provided, for example, as a layer containing aknown white coloring material (for example, a white pigment or thelike), a resin component, and the like.

Specific examples of the white coloring material include white pigmentssuch as titanium dioxide, zinc oxide, and calcium carbonate.

The white layer is a layer that does not contain the microcapsulesand/or electron-accepting compound described above.

In a case where the white layer is to be provided by coating thesubstrate with the composition for forming a white layer, for example,the composition for forming a white layer may be prepared, applied tothe substrate (for example, by means of coating), and dried. In thiscase, the same method as that used for forming the pressure-sensitivelayer can be used as a coating method.

In a case where the white layer is to be provided by sticking a sheet orfilm for forming a white layer, a desired sheet or film for forming awhite layer may be prepared and stuck onto the substrate by knownmethods (for example, sticking using an adhesive and the like).

-Easy Adhesion Layer-

The easy adhesion layer is preferably provided to improve theadhesiveness between the substrate and the pressure-sensitive layer.

In a case where the pressure measuring material of the presentdisclosure has an easy adhesion layer, it is preferable to adopt anaspect in which the material has at least a substrate, an easy adhesionlayer, and a pressure-sensitive layer provided in this order.

In a case where the pressure measuring material has an easy adhesionlayer and a white layer, it is preferable to adopt an aspect in whichthe material has a substrate, an easy adhesion layer, a white layer, anda pressure-sensitive layer provided in this order.

The easy adhesion layer is a layer that does not contain themicrocapsules and/or electron-accepting compound described above.

From the viewpoint of improving the adhesiveness between the substrateand the polymer matrix included in the pressure-sensitive layer, theeasy adhesion layer preferably contains a resin.

Examples of the resin include an acrylate resin, a urethane resin, astyrene resin, and a vinyl resin.

The easy adhesion layer may be a layer containing a urethane polymer, ablocked isocyanate, or the like.

The easy adhesion layer can be formed, for example, by bonding asubstrate and an easily adhesive sheet or film together, and coating thesubstrate with an easy adhesion layer-forming composition.

The thickness of the easy adhesion layer is not particularly limited,and can be selected depending on the purpose and the like.

The thickness of the easy adhesion layer is preferably 0.005 µm to 1.0µm, more preferably 0.005 µm to 0.5 µm, even more preferably 0.005 µm to0.2 µm, and still more preferably 0.01 µm to 0.1 µm.

(Thickness of Pressure Measuring Material)

The thickness of the pressure measuring material of the presentdisclosure is not particularly limited, and is preferably 10 µm to 800µm and more preferably 10 µm to 500 µm.

-Thickness T

A thickness T of a layer calculated by subtracting the thickness of thesubstrate from the thickness of the pressure measuring material ispreferably 1 µm to 250 µm, more preferably 3 µm to 200 µm, and even morepreferably 5 µm to 150 µm.

The thickness T can be measured by the same method as that used formeasuring the thickness t of the pressure-sensitive layer describedabove.

Specifically, the thickness T can be determined by measuring thethickness of the pressure measuring material and the thickness of thesubstrate at 10 sites that are randomly selected, calculating thedifference between the thickness of the pressure measuring material andthe thickness of the substrate, and calculating an arithmetic mean ofthe calculated value.

(Arithmetic Mean Roughness Ra of Pressure Measuring Material)

The arithmetic mean roughness Ra of the pressure measuring material ispreferably 10.0 µm or less.

The preferred range of the arithmetic mean roughness Ra in a case wherethe pressure-sensitive layer is a layer of the first aspect and thesecond aspect is as described above.

The arithmetic mean roughness Ra corresponds to the arithmetic meanroughness Ra of the surface opposite to the substrate side. The methodfor measuring the arithmetic mean roughness Ra is as described above.

(Ratio of Thickness T to Inner Diameter P of Microcapsules)

In the pressure measuring material, the ratio of the thickness T to theinner diameter p of the microcapsules, T/p, is preferably 1.2 or more,and more preferably 1.3 or more. In a case where T/p is 1.2 or more, thegradation in a high pressure range of 100 MPa or higher is furtherimproved. T/p is more preferably 1.2 to 5.

In a case where the pressure measuring material has thepressure-sensitive layer of the first aspect, the ratio of the thicknessT to the inner diameter p₁ of the microcapsules, T/p₁, is preferably 5or less, more preferably 1.2 to 5, and even more preferably 1.2 to 3.

In a case where the pressure measuring material has thepressure-sensitive layer of the second aspect, the ratio of thethickness T to the inner diameter p₂ of the microcapsules, T/p₂, ispreferably 5 or less, more preferably 1.2 to 5, and even more preferably1.3 to 5.

Matters Relating to Pressure Measurement

The pressure measurement using the pressure measuring material of thepresent disclosure can be performed by placing the pressure measuringmaterial at the site where pressure or pressure distribution is to bemeasured, and applying pressure to the pressure measuring material inthis state.

The pressure may be any of point pressure, linear pressure, or surfacepressure.

In a case where the pressure measuring material of the presentdisclosure is caused to develop color, a density difference (ΔD)calculated by subtracting the density of color developed by applying apressure of 2,000 MPa from the density of color developed by applying apressure of 1,000 MPa is preferably 0.6 or more.

In a case where ΔD is more than 0.6, the obtained pressure measuringmaterial of the present disclosure can provide more visible or readabledensity and can better reproduce density gradation in a case wherepressure is applied thereto for developing color.

In a case where the pressure measuring material having thepressure-sensitive layer of the first aspect is caused to develop color,a density difference (ΔD) calculated by subtracting the density of colordeveloped by applying a pressure of 1,000 MPa from the density of colordeveloped by applying a pressure of 2,000 MPa is preferably 0.1 or more,and more preferably 0.4 or more.

In a case where ΔD is 0.1 or more (preferably 0.4 or more), the obtainedpressure measuring material of the present aspect can provide morevisible or readable density and can better reproduce density gradationin a case where a pressure of 500 MPa or higher is applied thereto fordeveloping color.

In a case where the pressure measuring material having thepressure-sensitive layer of the second aspect is caused to developcolor, a density difference (ΔD2) calculated by subtracting the densityof color developed by applying a pressure of 100 MPa from the density ofcolor developed by applying a pressure of 500 MPa is preferably 0.1 ormore, and more preferably 0.4 or more.

In a case where ΔD2 is 0.1 or more (preferably 0.4 or more), theobtained pressure measuring material of the present aspect can providemore visible or readable density and can better reproduce densitygradation in a case where a pressure of 100 MPa to 500 MPa is appliedthereto for developing color.

The density of color developed is a value measured using a reflectiondensitometer (for example, RD-19I manufactured by GretagMacbeth GmbH).

In a case where pressure which is preferably 100 MPa to 10,000 MPa (morepreferably 100 MPa to 3,000 MPa) is applied to the pressure measuringmaterial of the present disclosure, the material exhibits properties inwhich the density of color developed increases as the pressureincreases, that is, gradation of color development.

For the pressure measuring material of the present disclosure, thegradation of color development is preferably properties in which thedensity of color developed linearly increases as the pressure increases(that is, the pressure and the density of color developed areproportional to each other).

For the pressure measuring material of the present disclosure, a rangeof pressure to be measured including the aforementioned range can be setdepending on the purpose of measurement. For example, in an aspect ofthe pressure measuring material of the present disclosure, the materialcan be used for measuring pressure in a range of 1,000 MPa or higher(for example, 1,000 MPa to 3,000 MPa, and preferably 1,000 MPa to 2,000MPa). Furthermore, in another aspect of the pressure measuring materialof the present disclosure, the material can be used for measuringpressure in a pressure range lower than 1,000 MPa (for example, 100 MPato 500 MPa).

The pressure measuring material of the present disclosure may be used,for example, for the following uses in various fields. However, the useof the material is not limited thereto. Some of the uses exemplifiedbelow overlap each other.

For example, the pressure measuring material of the present disclosuremay be used for manufacturing of vehicles such as automobiles oraircraft (for example, checking pressure distribution in molding ofvarious constituent members, bodies, and the like or assembly ofconstituent members), construction (for example, checking pressuredistribution in assembly of building materials), manufacturing ofelectronic products (for example, checking pressure distribution incurved surface machining (bonding of curved surface display or thelike)), transport (for example, checking impact force applied to cargodue to transport), metal processing (for example, checking mold contactin the manufacture of various metal products.), molding of resinproducts (for example, checking mold contact during molding of resinproducts), molding of pharmaceuticals (for example, checking pressuredistribution in making tablets.), furniture (for example, checkingpressure distribution within surface of furniture (such as the seat of achair, sofa, and the like), stationery (for example, checking grip forceapplied to writing materials and the like), sports equipment (forexample, checking impact force applied to an article (such as a ball)constituted with an elastic material), and the like.

EXAMPLES

Hereinafter, the present invention will be more specifically describedwith reference to examples. As long as the gist of the present inventionis maintained, the present is not limited to the following examples.Unless otherwise specified, “%” and “part” are based on mass.

Example 1 <Preparation of Microcapsule Solution (A) EncapsulatingElectron-Donating Dye Precursor>

As an electron-donating dye precursor, 10 parts of the followingcompound (A) was dissolved in 53 parts of linear alkylbenzene (JX NipponOil & Energy Corporation, grade alkene L, boiling point: 130° C. orhigher), thereby obtaining a solution A.

Then, 14 parts of synthetic isoparaffin (Idemitsu Kosan Co., LTD., IPsolvent 1620, boiling point: 130° C. or higher) and 0.4 parts ofN,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine (ADEKA CORPORATION,ADEKA polyether EDP-300) dissolved in 1.2 parts of ethyl acetate wereadded to the solution A being stirred, thereby obtaining a solution B.

Furthermore, 18 parts of a trimethylolpropane adduct (DIC Corporation,BURNOCK D-750) of tolylene diisocyanate dissolved in 3 parts of ethylacetate was added to the solution B being stirred, thereby obtaining asolution C.

The solution C was added to a solution obtained by dissolving 8 parts ofpolyvinyl alcohol (PVA-205, KURARAY CO., LTD., number-average molecularweight: 25,000, dispersant) in 110 parts of water, and the mixture wasemulsified and dispersed. Water (340 parts) was added to the emulsionobtained after emulsification and dispersion, and the mixture was heatedto 70° C. with stirring, stirred for 1 hour, and then cooled.

Water was further added to the cooled solution so that the concentrationwas adjusted, thereby obtaining a microcapsule solution (A)encapsulating an electron-donating dye precursor having a concentrationof solid contents of 25%.

The volume-based median diameter (D50) of the obtained microcapsules was11 µm.

The volume-based median diameter was measured using Microtrac MT3300EXII(manufactured by NIKKISO CO., LTD.).

<Preparation of Pressure Measuring Sheet (A)>

The microcapsule solution (A) (20 parts by mass) was mixed with 11 partsof a 40% dispersion of zinc 3,5-bis(α-methylbenzyl) salicylate which isan electron-accepting compound as a developer and 20 parts of a 20%aqueous solution of polyvinyl alcohol (PVA-105, KURARAY CO., LTD.number-average molecular weight: 22,000) as a specific polymer compoundforming a polymer matrix, thereby obtaining a pressure-sensitivelayer-forming composition (A).

By using a bar coater, a 75 µm thick PET substrate (A4300: manufacturedby TOYOBO CO., LTD.) was coated with the obtained pressure-sensitivelayer-forming composition (A) so that a 15 µm thick film was obtainedafter drying, and the composition was dried at 80° C., thereby obtaininga pressure measuring sheet (A) (pressure measuring material) having apressure-sensitive layer on a PET substrate.

A vertically cut cross section of the pressure measuring sheet (A) wasobserved with a desktop microscope “Miniscope TM3030Plus” (manufacturedby Hitachi High-Tech Corporation.), and the thickness of the pressuremeasuring sheet (A) and the thickness of the PET substrate at 10 randomsites were measured. Furthermore, the difference between the thicknessof the pressure measuring sheet (A) and the thickness of the PETsubstrate was calculated, and the arithmetic mean thereof was determinedand adopted as the thickness (film thickness) of the pressure-sensitivelayer. It was confirmed that the thickness (film thickness) of thepressure-sensitive layer was 15 µm.

The ratio (t/d) of the thickness t of the pressure-sensitive layer tothe particle diameter d of the microcapsules is 1.36.

The Martens hardness of polyvinyl alcohol (PVA-105, KURARAY CO., LTD.)forming a polymer matrix was measured using a “microhardness testerHM2000” manufactured by FISCHER instruments. For measurement, in alaboratory environment of 23° C. and 50% RH, first, a load ranging from0 mN to the maximum test load was applied for 10 seconds by using adiamond indenter (Berkovich indenter), then the maximum test load waskept for 5 seconds, and lastly a load ranging from the maximum test loadto 0 mN was applied for 10 seconds. The maximum test load was divided bythe indenter surface area at the maximum indentation depth, and theobtained value was adopted as a Martens hardness (N/mm²).

Under a maximum test load set to obtain a maximum indentation depth of0.5 µm, the measured Martens hardness of a 5 µm thick polymer matrix(PVA105) film formed on a glass substrate was 165 N/mm².

<Color Development Evaluation A>

The pressure measuring sheet (A) of Example 1 obtained as above was cutinto four samples having a length of 9 cm to 11 cm.

Any of the pressure shown in the column of Pressure in the followingTable 1 was applied to each of the samples. As a result, it wasconfirmed that the samples developed color due to the application ofpressure. Pressure was applied using a press machine (DSF-C1-A,manufactured by AIDA ENGINEERING, LTD.).

The density of color developed by the samples performing colordevelopment was measured using a spectrodensitometer (X-Rite 504manufactured by X-Rite, Incorporated.). The measurement results areshown in the column of Color density in Table 1.

FIG. 1 is a graph showing the relationship between the pressure and thedensity of color developed.

TABLE 1 Sample No. Pressure (MPa) Density of color developed 1 912 0.312 1,267 0.37 3 1,508 0.46 4 1,824 1.06

As shown in Table 1 and FIG. 1 , it has been confirmed that the pressuremeasuring sheet of Example 1 can perform color development withexcellent gradation in a high pressure range higher than 1,000 MPa.

Example 2

A pressure measuring sheet (pressure measuring material) of Example 2was prepared in the same manner as in Example 1, except that a part ofPVA-105 used in preparing the pressure measuring sheet (A) was replacedwith a polyol polyalkylene alkyl ether surfactant (NOIGEN LP-90,manufactured by DKS Co., Ltd.).

Examples 3 to 8

Pressure measuring sheets of Examples 3 to 8 were prepared in the samemanner as in Example 2, except that the composition of each material waschanged as shown in Table 2.

A microcapsule solution (B) was prepared as below.

<Preparation of Microcapsule Solution (B) EncapsulatingElectron-Donating Dye Precursor>

As electron-donating dye precursors, 6 parts of3′,6′-bis(diethylamino)-2-(4-nitrophenyl)spiro[isoindole-1,9′-xanthen]-3-one(Pink-DCF, manufactured by Hodogaya Chemical Co., Ltd.) and 8 parts of6′-(diethylamino)-1’,3′-dimethylfluorane (Orange-DCF manufactured byHodogaya Chemical Co., Ltd.) were dissolved in 70 parts of Hisol SAS-296(oil component (solvent) manufactured by Nippon Oil Corporation; amixture of 1-phenyl-1-xylylethane and 1-phenyl-1-ethylphenylethane),thereby obtaining a solution A2. Then, 19 parts of synthetic isoparaffin(Idemitsu Kosan Co., LTD., IP solvent 1620) and 0.7 parts ofN,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine (ADEKA CORPORATION,ADEKA polyether EDP-300) dissolved in 2.5 parts of methyl ethyl ketonewere added to the solution A2 being stirred, thereby obtaining asolution B2. Furthermore, 77 parts of a trimethylolpropane adduct (DICCorporation, BURNOCK D-750, containing 25% ethyl acetate) of tolylenediisocyanate dissolved in 6 parts of ethyl acetate was added to thesolution B2 being stirred, thereby obtaining a solution C2. The solutionC2 was added to a solution obtained by dissolving 10 parts of polyvinylalcohol (KL-318, KURARAY CO., LTD.) in 140 parts of water, and themixture was emulsified and dispersed. Water (200 parts) was added to theemulsion obtained after emulsification and dispersion, and the mixturewas heated to 70° C. with stirring, stirred for 1 hour, and then cooled.Water was added thereto to adjust the concentration, thereby preparing amicrocapsule solution (B) encapsulating an electron-donating dyeprecursor having a concentration of solid contents of 20% by mass. Themedian diameter of the microcapsules was 8 µm.

Comparative Example 1

According to Example 1 of JP2009-019949A, a two-sheet type pressuremeasuring sheet consisting of an electron-donating colorless dye sheetand a developer sheet was produced.

[Evaluation]

“Measurement of void volume” was performed on the pressure-sensitivelayers of the pressure measuring sheets of Examples 1 to 8.

In addition, the pressure measuring sheets of Examples 2 to 8 andComparative Example 1 was evaluated by the following “density gradationevaluation A” and “color development evaluation B”.

[Measurement of Void Volume]

Each pressure measuring sheet (PET films on which a pressure-sensitivelayer was formed) of Examples 1 to 8 was cut in 10 cm × 10 cm, and themass (m₁) thereof was measured. Then, diethylene glycol was applied to asurface provided with the pressure-sensitive layer and caused topermeate the material, the diethylene glycol remaining on the surfacewas wiped off, and then a mass (m₂) was measured. Then, X = m₂ - m₁ wascalculated, and the void volume was calculated from the followingequation. The density of the diethylene glycol is 1.118.

Void volume(m₁/m₂) = 100 × X ÷ 1.118

All the pressure measuring sheets obtained in Examples 1 to 8 had a voidvolume of 1 mL/m² or less.

[Density Gradation Evaluation A (1,000 MPa to 2,000 MPa)]

For the pressure measuring sheets of Examples 1 to 8 and ComparativeExample 1, the density of color developed at 1,000 MPa and the densityof color developed at 2,000 MPa were measured, and a difference ΔD1 wascalculated by subtracting the density of color developed at 1,000 MPafrom the density of color developed at 2,000 MPa and evaluated accordingto the following evaluation standard.

The pressure application method and the measurement device were the sameas those in Color development evaluation A.

Pressure measuring sheets graded “A” and “B” are within the acceptablerange for practical use, and pressure measuring sheets graded “A” arethe most excellent. The results are shown in Table 2.

<Evaluation Standard>

“A”: ΔD1 is 0.4 or more.

“B″: ΔD1 is 0.1 or more and less than 0.4.

“C”: ΔD1 is less than 0.1.

[Color Development Evaluation B]

The pressure measuring sheets of Examples 1 to 8 and Comparative Example1 were evaluated according to the following evaluation standard, basedon the measurement result of density of color developed at 1,000 MPathat was obtained in Density gradation evaluation A described above. Theresults are shown in Table 2.

<Evaluation Standard>

“A”: The density of color developed is 0.5 or more.

“B″: The density of color developed is less than 0.5.

TABLE 2 Form of sheet Amount in presence-sensative layer [%by mass]Physical properties Evaluation result Microcapsule Electron-acceptingcompound Specific polymer compound Surfactor Particle diameter d₁ ofcapsule Inner diameter p₁ of capsule Thickness T [µm] T/a₁ T/p₁ Capsulecontent [% by volume] R_(a) Density graduation evalaution A (1.000MPa-2.000 MPa) Color development evaluation B (1.000 MPa) Example 1Monosheet Solution (A) 34.4 Salicylate 32.8 32.8 0 11 10.5 15 1.36 1.435 1.0 A A Example 2 Monosheet Solution (A) 34.4 Salicylate 32.8 32.70.1 11 10.5 15 1.36 1.4 35 1.0 A A Example 3 Monosheet Solution (A) 49.2Salicylate 32.8 17.9 0.1 11 10.5 15 1.36 1.4 50 1.0 B A Example 4Monosheet Solution (A) 50.9 Salicylate 24.5 24.5 0.1 11 10.5 15 1.36 1.450 1.0 A A Example 5 Monosheet Solution (A) 38.7 Salicylate 30.6 30.60.1 11 10.5 20 1.8 1.9 40 1.0 Example 6 Monosheet Solution (B) 34.4Salicylate 32.8 32.7 0.1 8 6.9 15 1.9 2.2 35 1.0 A A Example 7 MonosheetSolution (B) 34.4 Salicylate 32.8 32.7 0.1 8 6.9 20 2.5 2.9 35 1.0 A AExample 8 Monosheet Solution (A) 34.4 Activated day 32.8 32.7 0.1 1110.5 15 1.36 1.4 35 1.0 B A Comparative Example 1 Two-sheet Solution(C) - Activated day - - - 20 19.8 - - - - - C A

In Table 2, T is a thickness of a layer calculated by subtracting thethickness of the substrate from the thickness of the pressure measuringmaterial, T/p₁ is the ratio of the thickness T to the inner diameter p₁of the microcapsules, T/d₁ is the ratio of the thickness T to theparticle diameter d₁ (median diameter) of the microcapsules, and Ra isthe arithmetic mean roughness of the outermost surface (surface of thepressure-sensitive layer) on the side opposite to the substrate. All ofthese are values obtained by the methods described above.

In Table 2, “-” means that the content of the corresponding component iszero or the corresponding item is not measured.

Example 9 -Preparation of Color Development-inducing Layer-FormingComposition-

A 10% by mass aqueous sodium hydroxide solution (10 parts), 750 parts ofwater, and 1 part of sodium hexametaphosphate (Nippon ChemicalIndustrial CO., LTD.) were added to 100 parts of activated clay: SILTONF-242 (MIZUSAWA INDUSTRIAL CHEMICALS, LTD.) as an electron-acceptingcompound, 100 parts of amorphous silica (MIZUSAWA INDUSTRIAL CHEMICALS,LTD., MIZUKASIL P-78A, inorganic particles not being anelectron-accepting compound), and the mixture was dispersed using ahomogenizer. This mixture was further mixed with 140 parts of a modifiedacrylic acid ester copolymer (ZEON CORPORATION, Nipol LX814,concentration of solid contents: 47%, specific polymer compound), 28parts of an anionic olefin-based resin (Arakawa Chemical Industries,Ltd., POLYMARON 482, concentration of solid contents: 25%, specificpolymer compound), 5 parts of a 15% aqueous solution of a side chainalkylbenzene sulfonic acid amine salt (DKS Co., Ltd., NEOGEN T), 35parts of a 1% aqueous solution of polyoxyethylene polyoxypropylenelauryl ether (DKS Co., Ltd., NEOGEN LP-70), and 35 parts of a 1% aqueoussolution ofsodium-bis(3,3,4,4,5,5,6,6,6-nonafluorohexyl)-2-sulfinatooxysuccinate(FUJIFILM Corporation, W-AHE), thereby preparing a colordevelopment-inducing layer-forming composition containingelectron-accepting compounds.

-Preparation of Color-Developing Layer-Forming Composition-

The 20% microcapsule solution (B) encapsulating an electron-donating dyeprecursor (70 parts) obtained as above was further mixed with 0.8 partsof an anionic olefin-based resin (Arakawa Chemical Industries, Ltd.,POLYMARON 482, concentration of solid contents: 25%, specific polymercompound), 3.1 parts of a polymer (Rohm and Haas Company, OROTAN 165A,concentration of solid contents: 21%, specific polymer compound), 0.5parts of a 15% aqueous solution of side chain alkylbenzene sulfonateamine salt (DKS Co., Ltd., Neogen T), 5 parts of a 1% aqueous solutionof polyoxyethylene polyoxypropylene lauryl ether (DKS Co., Ltd., NEOGENLP-70), and 5 parts of a 1% aqueous solution ofsodium-bis(3,3,4,4,5,5,6,6,6-nonafluorohexyl)-2-sulfinatooxysuccinate(FUJIFILM Corporation, W-AHE), thereby preparing a color-developinglayer-forming composition.

-Preparation of Pressure Measuring Material-

By using a bar coater, a 75 µm thick polyethylene terephthalate (PET)film (A4300: manufactured by TOYOBO CO., LTD.) was coated with the colordevelopment-inducing layer-forming composition obtained as above in asolid coating amount of coating amount of 20 g/m², thereby forming acolor development-inducing layer. Next, by using a bar coater, the colordevelopment-inducing layer formed by coating was coated with thecolor-developing layer-forming composition in a solid coating amount of3.5 g/m², thereby forming a color-developing agent layer.

In this way, a monosheet-type pressure measuring material was preparedwhich had a PET film as a substrate and a pressure-sensitive layer thatwas on the substrate and consisted of two layers, a colordevelopment-inducing layer and a color-developing agent layer, providedin this order.

Examples 10 to 14

Pressure measuring sheets of were prepared in the same manner as inExample 9, except that the composition of each material was changed asshown in Table 3.

Comparative Example 2

The same pressure measuring sheet as the two-sheet type pressuremeasuring sheet used in Comparative Example 1 described above was used.

[Evaluation]

“Measurement of void volume” was performed on the pressure measuringsheets of Examples 9 to 14.

In addition, the pressure measuring sheets of Examples 9 to 14 andComparative Example 2 were evaluated by the following “density gradationevaluation B” and “color development evaluation C”.

[Measurement of Void Volume]

The void volume of the pressure measuring sheets of Examples 9 to 14 wasmeasured in the same manner as in the pressure measuring sheets ofExamples 1 to 8 described above.

All the pressure measuring sheets obtained in Examples 9 to 14 had avoid volume in a range of 5 mL/m² to 20 mL/m².

[Density Gradation Evaluation B (100 MPa to 500 MPa)]

For the pressure measuring sheets of Examples 9 to 14 and ComparativeExample 2, the density of color developed at 100 MPa and the density ofcolor developed at 500 MPa were measured. A difference ΔD2 wascalculated by subtracting the density of color developed at 100 MPa fromthe density of color developed at 500 MPa, and evaluated according tothe following evaluation standard.

The pressure application method and the measurement device were the sameas those in Color development evaluation A.

Pressure measuring sheets graded “A” and “B” are within the acceptablerange for practical use, and pressure measuring sheets graded “A” arethe most excellent. The results are shown in Table 3.

<Evaluation Standard>

“A”: ΔD2 is 0.4 or more.

“B”: ΔD2 is 0.1 or more and less than 0.4.

“C”: ΔD2 is less than 0.1.

[Color Development Evaluation C]

For the pressure measuring sheets of Examples 9 to 14 and ComparativeExample 2, the density of color developed at 300 MPa was measured. Thepressure application method and the measurement device were the same asthose in Color development evaluation A.

Based on the obtained measurement results, evaluation was performedaccording to the following evaluation standard. The results are shown inTable 3.

<Evaluation Standard>

“A”: The density of color developed is 0.5 or more.

“B″: The density of color developed is less than 0.5.

TABLE 3 Form of sheer Amount in pressure-sensitive layer [% by mass]Physical properties Puslation result Coins-developing layer Colordevelopment-inducing layer Menoapsule polymer compound OthersElectron-accepting compound Specific polymer compound Silica othersThickness of color-developing layer [µm] Thickness of colordevelopment-inducing layer [µm] Thickness [µm] Particle diameter d₂ ofcapsule [µm] diameter p₂ of capsule [µm] T/d₃ T/P₂ Ra [µm] Densitygradation evaluationP ₁[00 MPa-500 MPa) Color development evaluation C(??MPa) Example 9 Monush eet Solution (B) 13.9 0.8 0.3 Activated clay30.8 22.4 30.8 1.0 3 20 23 8 6.9 2.83 3.36 4.0 A A Example 10 MonoshSolution (B) 27.8 1.5 0.6 Activated clay 25.3 18.3 25.3 1.0 6 20 27 86.9 3.38 3.94 A A Example 11 Monosh eet Solution (B) 27.8 1.5 0.6Activated clay 50.6 18.5 0.0 1.0 6 20 27 8 6.9 3.38 3.94 3.0 B A Example12 Monosh eet Solution (B) 34.8 1.9 0.8 Activated clay 22.6 16.5 22.6 720 27 8 6.9 3.38 3.94 1.3 B A Example 12 Monush eet Solution (B) 27.81.5 0.6 Activated clay 25.3 18.5 25.3 1.0 6 20 27 11 10.5 2.45 2.57 5.0A A Example 13 Monush eet Solution (B) 13.8 0.8 0.3 Activated clay 30.8322.4 30.8 1.0 8 20 23 11 10.5 2.09 2.19 4.0 A A Example 14 Monush eetSolution (B) 27.8 1.5 0.6 Salicyiate 25.3 18.5 25.3 1.0 6 20 27 8 6.93.38 3.94 3.0 B B Comparative Example 2 Two-side Solution (C) - - -Activated clay - - - - - - - 20 19.8 - - - C A

In Table 3, T is a thickness of the layer calculated by subtracting thethickness of the substrate from the thickness of the pressure measuringmaterial, T/p₂ is the ratio of the thickness T to the inner diameter p₂of the microcapsules, T/d₂ is the ratio of the thickness T to theparticle diameter d₂ (average particle diameter) of the microcapsules,and Ra is the arithmetic mean roughness of the outermost surface(surface of the pressure-sensitive layer) on the side opposite to thesubstrate. All of these are values obtained by the methods describedabove.

In Table 3, “-” means that the content of the corresponding component iszero or the corresponding item is not measured.

EXPLANATION OF REFERENCES

-   10, 20: pressure measuring material-   12, 22: substrate-   14, 24: pressure-sensitive layer-   24 a: color-developing layer-   24 b: color development-inducing layer-   15, 25: electron-accepting compound-   16, 26: polymer matrix-   18, 28: microcapsules-   18 a, 28 a: capsule wall-   18 b, 28 b: microcapsule inclusion (core material)

The entire disclosure of Japanese Patent Application No. 2019-006244,filed Jan. 17, 2019, is incorporated into the present specification byreference.

All documents, patent applications, and technical standards described inthe present specification are incorporated into the presentspecification by reference, as if each of the documents, the patentapplications, and the technical standards is specifically andindividually described.

What is claimed is:
 1. A pressure measuring material comprising: asubstrate; and a pressure-sensitive layer, wherein thepressure-sensitive layer contains a polymer matrix containing a polymercompound having a molecular weight of 1,000 or more, microcapsulesencapsulating an electron-donating dye precursor and a solvent, and anelectron-accepting compound, wherein the pressure-sensitive layer has acolor development-inducing layer having the electron-accepting compoundand the polymer matrix and a color-developing layer having themicrocapsules, and a thickness of the color-developing layer is equal toor less than ½ of a thickness of the color development-inducing layer.2. The pressure measuring material according to claim 1, wherein adensity difference (Δ D2) calculated by subtracting the density of colordeveloped by applying a pressure of 100 MPa from the density of colordeveloped by applying a pressure of 500 MPa is 0.1 or more.
 3. Thepressure measuring material according to claim 1, wherein theelectron-accepting compound includes acidic clay or activated clay. 4.The pressure measuring material according to claim 1, wherein theelectron-accepting compound includes a metal salt of salicylic acid. 5.The pressure measuring material according to claim 1, wherein thepressure-sensitive layer has inorganic particles other than theelectron-accepting compound.
 6. The pressure measuring materialaccording to claim 1, wherein the arithmetic mean roughness Ra of theoutermost surface opposite to the substrate is 10.0 µm or less.
 7. Thepressure measuring material according to claim 6, wherein the arithmeticmean roughness Ra of the outermost surface opposite to the substrate is2.0 µm to 10.0 µm.
 8. The pressure measuring material according to claim1, wherein a void volume is 5 mL/m² to 20 mL/m².
 9. The pressuremeasuring material according to claim 1, wherein in a case where Trepresents a thickness of a layer calculated by subtracting a thicknessof the substrate from a thickness of the pressure measuring material,and p represents an inner diameter of the microcapsules, a ratio T/p is1.2 or more.
 10. The pressure measuring material according to claim 9,wherein in a case where T represents the thickness of a layer calculatedby subtracting a thickness of the substrate from a thickness of thepressure measuring material, and p represents the inner diameter of themicrocapsules, the ratio T/p is 1.2 to 5.0.
 11. The pressure measuringmaterial according to claim 1, wherein a content of the polymer compoundhaving a molecular weight of 1,000 or more is 10% by mass or more withrespect to a total mass of the pressure-sensitive layer.
 12. Thepressure measuring material according to claim 1, wherein the substrateis a polyethylene terephthalate substrate or a polyethylene naphthalatesubstrate.
 13. The pressure measuring material according to claim 1,further comprising: an easy adhesion layer between the substrate and thepressure-sensitive layer.
 14. The pressure measuring material accordingto claim 1, wherein a wall material of the microcapsules includes atleast one kind of material selected from polyurethane urea orpolyurethane.